Drug-induced liver injury (DILI) is a significant cause of anti-tubercular therapy failure. Rifampicin and isoniazid co-treatment lead to cholestatic liver injury associated with the accumulation of protoporphyrin IX (PPIX), a heme precursor, in a human pregnane X receptor (hPXR)?dependent manner in mice. Therefore, the activation of hPXR by rifampicin can lead to DILI via PPIX accumulation; thus, antagonism of hPXR can prevent the clinical consequences of DILI. This project aims to evaluate a method of preventing DILI and to increase the understanding of the mechanisms of DILI. Upon completion of this project, I will elucidate a novel role of isoniazid in the development of DILI and provide an evaluation of a novel therapeutic agent in reducing the pathologic markers associated with DILI. This information may eventually be used to reduce the incidence and extent of anti-tubercular DILI in humans. Using an hPXR transgenic mouse model and primary human hepatocytes, I will evaluate the potential of a novel small-molecule inhibitor of hPXR that was developed in our laboratory to prevent liver injury associated with rifampicin and isoniazid treatment. PPIX accumulation will be assessed by LC/MS/MS in mice; hPXR target gene expression will be evaluated by performing western blot and qRT-PCR analyses of human primary cells and humanized mice. Liver injury will be evaluated by analyzing mouse liver enzyme activity in sera and via histopathologic examination. ALAS1/Alas1 (rate-limiting enzyme of heme synthesis) is induced in response to rifampicin and isoniazid (a ligand of hPXR) in both hPXR transgenic mice and primary human hepatocytes. This induction is modulated by co-treatment with an antagonist of hPXR. PPIX is increased in the liver of hPXR mice treated with both rifampicin and isoniazid. FECH/Fech is degraded in response to isoniazid. This degradation may be mediated by iron chelation of a metabolite of isoniazid. Using western blot analysis of primary human hepatocytes and human cancer cell lines ectopically overexpressing FECH, I will evaluate the potential role of an isoniazid metabolite in the degradation of FECH. Antagonism of hPXR with a small molecule after early detection of liver injury may be a viable therapeutic strategy to reduce the induction of ALAS1 and subsequent accumulation of PPIX resulting in liver injury. The successful completion of this project by 1 graduate student with 3+ years of experience should require approximately 1 year to complete and provide experience in the fields of cell biology, molecular biology, and pathology.